A hydrocarbon-fueled engine such as, for example, an internal combustion engine for a vehicle, may combust a mixture of air and fuel to drive mechanical equipment and perform work. The hot exhaust gas generated by the engine generally contains unwanted gaseous emissions and possibly some suspended particulate matter that may need to be converted to more innocuous substances before being discharged to the atmosphere. The gaseous emissions primarily targeted for abatement include carbon monoxide, unburned and partially burned hydrocarbons (HC's), and nitrogen oxide compounds (NOX) comprised of NO and NO2 along with nominal amounts of N2O. An exhaust aftertreatment system that includes specially catalyzed flow-through components may be employed to dynamically treat a continuous exhaust flow with variable concentrations of these emissions. Many different exhaust aftertreatment system designs have been developed. But in general these systems seek to oxidize both carbon monoxide and HC's (to carbon dioxide and water) and reduce NOX (to nitrogen and water). Suspended particulate matter, if present, is usually captured by a filter and burned off at regular intervals.
The catalytic conversion efficiency of carbon monoxide, HC's, and NOX over various types of catalysts depends largely on the air to fuel mass ratio of the mixture of air and fuel fed to the engine. A stoichiometric mixture of air and fuel (air to fuel mass ratio of about 14.7 for standard petrol-based gasoline) combusts to provide the exhaust flow with a reaction balance of oxidants (O2 and NOX) and reductants (CO, HC's, and H2). This type of exhaust flow composition is generally the easiest to treat. A conventional three-way-catalyst (TWC) that includes a platinum group metal mixture dispersed on a base metal oxide support material, and which is close-coupled to the engine, can simultaneously reduce NOX and oxidize carbon monoxide and HC's through various coupled catalytic reactions. But a stoichiometric mixture of air and fuel is not always maintained or even practical (i.e., a diesel engine). The engine may, for instance, combust a lean mixture of air and fuel (air to fuel mass ratio above 14.7 for standard petrol-based gasoline) to achieve more efficient fuel economy. The excess air contained in a lean mixture of air and fuel increases the concentration of uncombusted oxygen and decreases the concentrations of the various reductants in the exhaust flow. The catalytic reduction rate of NOX to N2 is slowed in such an oxidative environment over a conventional TWC and may require an entirely different system design or supplemental NOX treatment capacity to bring NOX concentrations within acceptable levels.
The two most prevalent approaches, to date, for reducing NOX in an oxygen enriched exhaust flow are a selective catalytic reduction (SCR) system and a lean NOX trap (LNT). A SCR system introduces a reductant such as ammonia (or urea because it reacts to form ammonia) or a hydrocarbon into the exhaust flow which, in turn, reacts with NOX in the presence of oxygen over a reaction-specific SCR catalyst to form nitrogen. A LNT directs the exhaust flow over a NOX absorption catalyst that stores NO2 as a nitrate species until purged with a source of reductants that also converts the desorbed NOX into nitrogen over a NOX reduction catalyst. The overall NOX conversion efficiency for both practices can be enhanced by decreasing the molar ratio of NO to NO2 in the NOX gas constituency originally produced by the engine. A preferred NO:NO2 molar ratio for rapid NOX reduction in the SCR system is approximately 1.0 (equimolar). A preferred ratio of NO:NO2 for the LNT is much less. Most, if not all, of the NO present in the exhaust flow is preferably oxidized to NO2 to maximize the NO2 absorption selectivity of the NOX absorption catalyst.
The NOX generated by the engine during combustion of a lean mixture of air and fuel generally constitutes greater than 90 mol % NO and less than 10 mol % NO2. An oxidation catalyst that can selectively oxidize NO to NO2 may be provided upstream of the SCR catalyst or the NOX absorption catalyst and, if desired, in close proximity to the hydrocarbon-fueled engine. The oxidation catalyst may be part of a diesel oxidation catalyst (DOC) or some other suitable two-way catalyst composition. The upstream oxidation catalyst oxidizes NO (to NO2) to achieve a more desirable NO:NO2 molar ratio and, additionally, may oxidize CO and HC's to some extent. The lower NO:NO2 molar ratio boosts NOX reduction activity in the SCR system or the LNT and, in turn, enhances the overall NOX conversion efficiency of the exhaust aftertreatment system. The oxidation catalyst may also be intermingled within the SCR catalyst or the NOX absorption and/or reduction catalysts to further oxidize NO that may slip past the upstream oxidation catalyst to ensure near-complete conversion of NOX to N2. Other oxidation catalysts that are more selective towards CO and HC's may be combined with the oxidation catalyst that affects NO to form a multi-functional catalyst material.
The oxidation catalyst that has conventionally been used in an exhaust aftertreatment system to oxidize NO to NO2 is fine particles of platinum or a platinum-based metal alloy. But platinum and platinum-based alloys are rather expensive and tend to suffer from poor thermal durability. A better-performing, lower-cost, and more durable oxidation catalyst that exhibits a useful NO to NO2 oxidative activity would be a valuable contribution to those interested in NOX treatment because it could serve as partial or total substitute for platinum and platinum-based alloys in an exhaust aftertreatment system.